The invention concerns a wire section of a machine for making fibrous material webs of the type with two continuous wires forming together a twin-wire zone, and with a dewatering element which can be pressed on the inside of the wire with at least one surface. The dewatering element features in the direction of travel of the fiber suspension, in interaction with the wires, two dewatering zones. The first dewatering zone is defined by an essentially flat surface, while a second dewatering zone is defined by a surface curving about an axis of curvature in the direction of travel of the fiber suspension. Coordinated with the dewatering element are at least two support axes on which support elements are arranged. Also, coordinated with the dewatering element is a pivot axis.
Twin-wire formers of this type are known from
(1) DE 40 05 420, PA1 (2) EP 0 397 430.
The twin-wire formers cited in these documents feature among others a dewatering unit consisting of a number of rigid slats bearing on the inside of the top wire and of flexibly arranged slats which are arranged on the inside of the bottom wire, essentially in the intermediate range of the slats bearing on the top wire.
The slats bearing on the inside of the top wire are preferably arranged on the underside of a dewatering box. The dewatering box is preferably equipped with a suction system for removal of the liquid issuing upwardly out of the fiber suspension. This box, or carrier for the slats bearing on the inside of the top wire, is coupled to essentially vertically movable support elements through which an adjustment, or positional change, of the box and also the positional change of the top wire relative to the bottom wire, for adaptation to different layer thicknesses, can be accomplished, since normally the top wire always is being moved along with the box. That is, the area swept by the slats on the inside of the top wire remains maximally constant.
The arrangement of the slats on the underside of the dewatering box, or on a carrier, can be subdivided in two areas, in keeping with the profile contour deriving thereof. In cooperation with at least one continuous wire, one speaks of two dewatering zones. Viewed in profile, the row of slats can describe an essentially straight stretch and/or a curved stretch. The underside of the dewatering box, or carrier, can then as well have this shape. A straight slat arrangement is often combined with a curved one.
In DE 40 05 420, the box features in its upper area relative to the inside of the top wire, e.g., pivot bearings which are coupled to essentially vertically movable support elements. The position of the pivot bearings is chosen such that two first ones, in the direction of travel of the fiber suspension, are arranged in the area of the front, lateral outside edge of the box and two second ones in the area of the rear outside edges of the box. The front and rear pivot bearings are situated each on a pivot axis. A sole positional change of the pivot axis of the front pivot bearings, i.e., adjustment of the front support elements, causes the dewatering box to pivot, or tilt, about the pivot axis of the rear pivot bearings.
The twin-wire former known from EP 0 397 430 comprises an upper and lower wire loop which together form within an area a twin-wire zone. Within this twin-wire zone there is a dewatering unit provided, which serves to dewater the fiber suspension. Here, too, the dewatering unit is formed by an arrangement of slats, with a number of rigidly arranged slats working against the inside of the top wire, whereas a number of flexibly arranged slats work in staggered fashion against the inside of the bottom wire.
The slats bearing on the inside of the top wire describe in profile, relative to the direction of web travel, a straight stretch with a following curvature. Consequently, the underside of the top wire box, or slat support element, may be of a design analogous to this shape.
Due to the necessity of changing the slat pressure for adaptation to different layer thicknesses, the upper part of the dewatering unit tilts about a pivot axis which is situated either exactly on the section line of the separating plane of the two dewatering zones with the underside of the upper part of the dewatering unit, or is situated in the curved area of this part of the dewatering unit parallel to the separating plane between the dewatering zone with straight and curved design, on the curved surface of the underside of the upper part of the dewatering unit.
The disadvantage of these embodiments cited in DE 40 05 420 and EP 0 397 430, at layer thickness change, is constituted by the change of the departure line of the wires from top wire dewatering box, or support element for the rigid slats, which is associated with the deflection, and thus also by the change of the approach angle to the following suction separator, or to other wire guide units. Depending on the pivotal direction of the dewatering unit, the wires run over the edge of the last slat in the direction of web travel, or they run in such a fashion over the edge of a slat arranged, e.g., on the underside of the upper part of the dewatering the unit which, in the direction of web travel, is arranged essentially before the last slat, in a fashion such that not all of the suction slots of the upper part of the dewatering unit are covered anymore. That is, the slats in the area not covered by the wire are then situated on a shank of an acute angle which is defined by the underside of the upper part of the dewatering unit and a plane which in this area is described by the course of the wires, or the upper wire. Deposits may accumulate in this acute angle, which in the subsequent lowering, or restoration, of the upper part of the dewatering unit may lead to damage. These depositions have a negative effect also on the process of dewatering the fiber suspension.
The problem underlying the invention is to fashion the arrangement and positional change options of a dewatering element in such a way that the said disadvantages will be avoided, i.e., that the rear part of the dewatering element in the direction of travel, which describes a curved surface, performs in the adaptation of the entrance gap on the dewatering element to different layer thicknesses, by positional change of the dewatering unit, an only negligible up and down motion. That is, that for instance with a top wire dewatering box the top wire will not essentially depart from the last slat, or that the top wire will by the upper wire dewatering box not be pulled into a certain, undesirable angle. Moreover, the entrance gap is to allow a simple adjustment and maintenance.